Add randomized tests for adding/removing keys

[Manav-Aggarwal]

Reference comment: #8 (comment)

We'd like to add randomized tests for Set and Remove that add new keys and delete existing keys respectively. Mainly, we'd like to make sure that Deep Subtrees are constructed with all the data they might need to rebalance correctly when new keys are added or existing keys are removed.

Light fuzzing in the tests might be useful as well.

Suggested tests:

• Batch-add random keys.
• Randomize both the operations, Set and Remove, and the keys they are operating on so calls to both are interleaved together in no particular order.

Make sure the tree rebalances correctly at each stage (check against existing IAVL tree: mutable tree).

Overall, the goal of this issue is to have strong substantial tests for an IAVL Deep Subtree.